Project Summary Water fluoridation is among the most noteworthy public health practices developed in the US. While never a federally-mandated policy, the strong scientific and rhetorical support for fluoridation by the Public Health Service, the American Dental Association, and numerous other scientific and professional bodies has helped overcome skepticism and opposition, and today some 70% of the nation's water supply is fluoridated. While dental decay remains a problem, the caries rate has declined significantly since the 1950s, a development that most public health officials and dentists attribute to water fluoridation. With US government support, water fluoridation became standard practice in a handful of other nations, including Australia, New Zealand, Canada, Ireland, and Brazil. These countries also exhibit strong government and scientific support for the practice, which was necessary in order to overcame significant opposition among a vocal minority of scientists and activists. Many European nations, however, were unable to overcome such opposition, and as a result have never fluoridated their water. The fact that the majority of them have nonetheless achieved the same rate of decline in dental caries as fluoridating countries suggests that this is a more complicated story than a simple triumphalist public health narrative would suggest. A rigorous historical analysis of fluoridation and anti-fluoridation efforts throughout the world can help us better understand the successes and limitations of fluoridation as a public health policy, as well as shedding light on the socio-political dynamic that has driven similar public health disputes? such as vaccination?since the Second World War. By examining how scientific ideas about fluoridation evolved and changed over time?and were received differently in different places? my work will be a significant contribution, not only to the history of fluoridation, but to scholarship on public health, scientific controversies, and the history of dentistry.